Carrie Johnson has said she was hospitalised for nearly a week with flu and pneumonia amid a surge in cases hitting the NHS.
The mother-of-three said she was admitted after struggling to breathe properly and urged her Instagram followers to "get the bloody flu jab".
Sharing a photograph from her hospital bed on social media, Mrs Johnson, 36, told of how she was hospitalised after struggling with a chest infection for over two weeks at home "over Christmas".
The wife of Boris Johnson, a former prime minister, has since been discharged, but added that she had not yet fully recovered.
Her hospital stay comes nearly five years after her husband was hospitalised after becoming infected with Covid-19 and treated in an intensive care unit.
Writing on Instagram, Mrs Johnson wrote: "I didn't expect to spend the first week of 2025 in hospital.
"After having a nasty chest infection for nearly 18 days at home over Christmas, it just got out of hand and I was struggling to breathe properly.
"Hospital confirmed I had flu and pneumonia.
"I was there nearly a week and I'm still not recovered. It could take another few weeks until I feel like myself again."
Mrs Johnson went on to thank NHS doctors and nurses, who she described as "the best people on earth".
She added: "I say it a lot but they have looked after me and my family when we've needed it most and I will never not be enormously grateful.
"They are the best of us."
UK hospitals are currently treating soaring numbers of flu patients as the NHS faces what is on course to be the worst season for the virus in more than a decade.
There were an average of 5,407 patients in hospital with flu every day over a week earlier this month, a 21 per cent rise on the week before and almost four times higher than the same week last year, according to official data.
The number was just 34 short of the peak recorded in 2022/23, which is expected to be surpassed. The previous high came in 2011/12.
Alongside flu, Covid and respiratory syncytial virus (RSV) - a common cause of coughs and colds - have also been driving an increase in admissions.
MPs have written to Wes Streeting, the Health Secretary, asking for his action plan to help the 14 hospitals that have declared critical incidents under winter pressure, according to the Guardian.
Mrs Johnson said one nurse at the John Radcliffe Hospital in Oxford, where she was admitted, "serenaded" her when she was feeling "particularly low".
Concluding her post, she wrote: "Obviously check with your doctor but my strong advice is to get the bloody flu jab.
"I really, really wish I had. It totally slipped my mind this year."